Optical disk recorders, in particular, employ objective lens having very small size and mass. Typically, such objective lenses are slideably mounted on a moveable support such that the beam passing through the objective lens is appropriately focused and positioned with respect to a record member. Generally, the acquisition of focus i.e., the movement of the objective lens to an in-focus position wherein the laser or other light beam is appropriately focused at the recording level, should be reliably accomplished for preventing retrys and delays in using an optical disk recorder. Also the best focus should be quickly and accurately obtained. It is desired also to ensure that the control of the focus acquisition is simplified for reducing costs of the optical system.
One of the problems involved in accurately acquiring focus is that of stiction. That is, as the objective lens is urged toward the in-focus position from an out of focus position, the lens may be subjected to undesired friction causing undesired and erratic motions in the focus acquisition process. Such undesired changes in friction reduces the probability of rapidly and accurately acquiring focus. It is desired to provide a simple system which aviates the stiction problem and provides accurate focus acquisition at low cost.